Note. SocDem – sociodemographic information, PAF10 – premenstrual assessment form 10 item version; PAF20 – premenstrual assessment form 20 item version; BDI-II – Beck’s Depression Inventory; BDI-FS – Beck’s Depression Inventory short version; ANT-R – attention network test-revised; DASS – Depression Anxiety Stress Scale; HRV ECG – heart rate variability measurement in lab (electrocardiography); HRV basic – measurement at rest at home on phone (photoplethysmography); W – week.
2.3 Smartphone-based heart rate variability biofeedback
The HRVB intervention used in this study was app-based and built with the software development kit (SDK) provided by Kenkou GmbH. The app measured HRV via photoplethysmography (PPG), whereby participants placed their index finger on the camera lens and a flash was used to illuminate the tissue. The camera measured the intensity of blood flow, and a peak detection algorithm was used to detect heartbeats.
Before each biofeedback session, participants underwent a 1-minute baseline measurement to assess their current state, which allowed the feedback to be adjusted accordingly. During the biofeedback session, participants saw a growing and shrinking circle indicating the breathing rhythm at 0.1 Hz. A fixed sinus-like wave was also displayed at the frequency of the paced breathing, and the pulse rate over time was mapped on top of this. Participants were instructed that the two waves would converge more as they relaxed. After the HRVB exercise, the user was given feedback on HRV improvement during the exercise to increase motivation. Screenshots of the app can be seen in Figure 2.
Figure 2. Screenshots of the HRVB application